The present invention relates to an electron gun for a cathode ray tube and a cathode ray tube employing the same, and particularly to an electron gun with reduced astigmatism and curvature of field for a cathode ray tube and a high definition cathode ray tube employing the same.
An electron gun used for picture tubes and display tubes needs to suitably control the shape of an electron beam spot on a screen area in accordance with the amount of deflection for obtaining a good focus characteristic and high resolution over the entire screen area.
The electron gun of this type has been disclosed, for example, in Japanese Patent Laid-open No. Hei 7-161309.
FIG. 7 is a schematic axial section view illustrating a configuration example of the prior art electron gun for a cathode ray tube disclosed in the above-described document; and FIGS. 8A and 8B are perspective views showing an electrostatic quadrupole electrode configuration shown in FIG. 7. In these figures, reference character K indicates a cathode; reference numeral 1 is a first electrode; 2 is a second electrode; 3 is a third electrode; 4 is a fourth electrode composed of a first sub-electrode 41 and a second sub-electrode 42; 5 is a fifth electrode; 6 is a sixth electrode; and 7 is a shield cup. Reference numeral 4-1 indicates a horizontal correction plate provided on the first sub-electrode 41 on the second sub-electrode 42 side thereof; 4-2 is a vertical correction plate provided on the second sub-electrode 42 on the first sub-electrode 41 side thereof; and 51 and 61 are astigmatism correction plates provided within the fifth electrode 5 and the sixth electrode 6, respectively.
Referring to FIG. 7, the fifth electrode 5 functions as a lower-potential electrode and the sixth electrode 6 functions as a higher-potential electrode, and a main lens is formed in a region between the fifth electrode 5 and the sixth electrode 6. The astigmatism correction plates 51 and 61 are disposed within the fifth electrode 5 and the sixth electrode 6 which form the main lens, respectively, and a single horizontally elongated opening is provided at each of opposing ends of the fifth electrode 5 and the sixth electrode 6.
In this electron gun, an electrostatic quadrupole lens for correcting deflection defocusing is formed in a region between the first sub-electrode 41 and the second sub-electrode 42 which form the focus electrode 4.
The electrostatic quadrupole lens is formed of the horizontal correction plates 4-1 provided on the first sub-electrode 41 on the second sub-electrode 42 side thereof (see FIG. 8A) and the vertical correction plates 4-2 provided on the second sub-electrode 42 on the first sub-electrode 41 side thereof (see FIG. 8B), and it is disposed as shown in FIG. 7.
The horizontal correction plates 4-1 are composed of a pair of approximately rectangular plates disposed in such a manner as to vertically sandwich an electron beam aperture in the first sub-electrode 41, and the vertical correction plates 4-2 are composed of a plurality of approximately rectangular plates disposed in such a manner that the two adjacent plates horizontally sandwich each electron beam aperture in the second sub-electrode 42.
A fixed focus voltage Vf.sub.1 is applied to the third electrode 3 and the second sub-electrode 42, and an AC voltage dVf varying in synchronization with the amount of deflection of electron beams and superposed on a fixed voltage Vf.sub.2 is applied to the first sub-electrode 41 and the fifth electrode 5.
In this way, the curvature of field and astigmatism caused by deflection of electron beams have been corrected by the use of two focus voltages, one constant voltage Vf.sub.1 applied to the third electrode 3 and the second sub-electrode 42 and another constant voltage Vf.sub.2 applied to the first sub-electrode 41 and the fifth electrode 5, and a dynamic voltage dV.sub.f superposed on the voltage Vf.sub.2.
In the electron gun disclosed in the above-described document, Japanese Patent Laid-open No. Hei 7-161309, as shown in FIG. 9, the constant voltage Vf.sub.1 is set to be considerably larger than the constant voltage Vf.sub.2 so that the differential voltage (Vf.sub.1 -Vf.sub.2) exceeds at least the maximum value of the voltage dV.sub.f.
With this arrangement, when the dynamic voltage dV.sub.f is increased, that is, when the amount of deflection of electron beams becomes larger, the strength of the electrostatic quadrupole lens formed between the first sub-electrode 41 and the second sub-electrode 42 becomes weak so that a lens action of the main lens stronger in the horizontal direction than in the vertical direction remains to correct the astigmatism. At the same time, differential potentials in the main lens, in a curvature-of-field correction lens formed between the third electrode 3 and the first sub-electrode 41, and in a curvature-of-field correction lens formed between the second sub-electrode 42 and the fifth electrode 5 become smaller, to lower the lens strength. Accordingly, a focusing force on deflected electron beams becomes weak, to thereby correct curvature of field.
With this electrode configuration and electrical configuration, a plurality of the curvature-of-field correction lenses can be obtained and thereby the sensitivity of the quadrupole lens can be increased, thereby making it possible to reduce the amplitude of the dynamic voltage and hence to suppress the increased circuit cost and the like.
With this configuration, astigmatism is corrected by controlling the cross-sectional shape of deflected electron beams, to thereby provide a color cathode ray tube having higher resolving power.
Ranges and typical values of voltages applied to the electron gun for a cathode ray tube shown in FIG. 7 are as follows:
______________________________________ ranges typical values ______________________________________ Eb 20 kV-40 kV 30 kV Vf1 4 kV-12 kV 8.4 kV Vf2 3 kV- 11 kV 7.6 kV dVf 0 V- 2 kV 0-800 V Ec2 200 V- 2 kV 750 V Ec1 -50 V- 50 V 0 V ______________________________________
In the above-described prior art, the fifth electrode is disposed adjacent to but spaced from the final accelerating electrode. The focus electrode adjacent to but spaced from the fifth electrode is divided into the first sub-electrode and the second sub-electrode to form a non-axially-symmetric lens or a lens non-circular in cross-section between the first sub-electrode and the second sub-electrode. A voltage varying in synchronization with deflection of electron beams is applied to the first sub-electrode and the fifth electrode, to deform the cross-sectional shape of an electron beam, thus correcting astigmatism due to deflection of the electron beam. At the same time, the lens strength of the main lens is changed in synchronization with deflection of electron beams by the fifth electrode and also the strengths of the curvature-of-field correction lenses between the third electrode and the first sub-electrode and between the second sub-electrode and the fifth electrode are changed, to correct curvature of field at a periphery of the screen.
In a color cathode ray tube, generally, a final accelerating voltage Eb, which is the maximum voltage in a range of from 20 to 40 kV, is supplied from an anode button embedded in a funnel, and other voltages are supplied from stem pins provided in a stem. An electron gun most typically used for a cathode ray tube needs to be supplied with, in addition to the final accelerating voltage Eb, eight kinds of voltages: a focus voltage Vf in a range of from 3 to 12 kV; a voltage Ec2 to be applied to the second electrode in a range of from 200 to 2 kV; a voltage Ec1 to be applied to the first electrode in a range of -50 to 50 V; positive and negative poles of a heater voltage Ef in a range of 4 to 8 V; and voltages EkR, EkG and EkB corresponding to three color cathodes in a range of from 0 to 250 V, and consequently the electron gun requires at least eight stem pins. FIG. 10 is a schematic view, seen from a phosphor screen side, showing an arrangement of stem pins typically used for color cathode ray tubes of a neck diameter of about 29 mm. In this arrangement, ten stem pins are arranged on a circumference of about 7.5 mm in radius, through which ten kinds of voltages can be applied to an electron gun at maximum. A pin 101 is supplied with a focus voltage V.sub.f ; pins 104 and 105 are connected to each other within the cathode ray tube and are supplied with a voltage Ec1; pins 106, 108 and 1011 are supplied with voltages EkG, EkR and EkB, respectively; a pin 107 is supplied with a voltage Ec2; and pins 109 and 1010 are connected with positive and negative poles of a voltage Ef, respectively. With this stem arrangement, a withstand voltage between the adjacent stem pins is about 2 kV at maximum in consideration of manufacturing variations and reliability in operation. Each pin, other than the pin 101, is supplied with only 2 kV at maximum, and accordingly, it sufficiently ensures electrical insulation from the adjacent pins, but the pin 101 is supplied with the V.sub.f in a range of from 3 to 12 kV and thereby two pin locations are not utilized on both sides of the pin 101 to ensure electric insulation. However, for the prior art electron gun described with reference to FIG. 7, the focus voltage Vf.sub.2 +dV.sub.f varying in synchronization with beam deflection in a range of from 3 to 12 kV needs to be supplied as well as the fixed focus voltage Vf.sub.1 of from 4 to 12 kV. As a result, the stem pin arrangement shown in FIG. 10 could not supply the two kinds of the voltages Vf.sub.1 and (Vf.sub.2 +dV.sub.f). Namely, to supply the two voltages Vf.sub.1 and Vf.sub.2, a special stem shown in FIG. 11 was used, in which the pins 111 and 112 are spaced wider than the regular spacing from the other pins. In the stem pin arrangement shown in FIG. 11, the pin 111 is supplied with the fixed focus voltage Vf.sub.1 and the pin 112 is supplied with the voltage (Vf.sub.2 +dV.sub.f) varying in synchronization with beam deflection, and the other pins are supplied with the voltages in the same manner as in the stem arrangement in FIG. 10, that is, the pin 115 is supplied with the voltage Ec1 of from -50 to 50 V; the pins 116, 118 and 1111 are respectively supplied with the voltages EkG, EkR and EkB in a range of from 0 to 250 V; the pin 117 is supplied with the voltage Ec2 of from 200 V to 2 kV; and the pin 119 and 1110 are respectively supplied with the positive and negative poles of the voltage Ef in a range of from 4 to 8 V. In addition, the replacement of the stem pin arrangement shown in FIG. 10 with that shown in FIG. 11 causes a problem that a socket receiving the stem pins needs to have a special structure, resulting in the increasing cost.